There have been a number of 18-inch torpedoes in service with the United Kingdom.

These have been used on ships of the Royal Navy and aircraft of both the Fleet Air Arm and Royal Air Force, while Royal Navy surface ships and submarines use 21-inch torpedoes.

The British "18-inch" torpedoes were 17.72 inches (45.0 cm) in diameter, beginning with the "Fiume" Whitehead torpedo of 1890.

45 cm "Fiume" (Whitehead) torpedo

First introduced into British service in 1894.^[1]

• Year : about 1888
• Weight : 845 lb (383 kg)
• Length : 11 ft 8 in (3.56 m)
• Warhead : 118 lb (54 kg) wet gun-cotton
• Performance : 26.5 knots (49.1 km/h; 30.5 mph) for 800 yd (730 m)
• Propulsion : Compressed air

Mark V

Used on the River-class and 1905 Tribal-class destroyers.

• Year : about 1899^[1]
• Weight : 1,353 lb (614 kg)
• Warhead : 296 lb (134 kg) including pistol
• Propulsion : Compressed air

Mark VI

Used on destroyers of the early 1900s.

• Year : about 1904^[1]
• Performance : 28.5 knots (52.8 km/h; 32.8 mph) for 4,000 yd (3,700 m) or 41 knots (76 km/h; 47 mph) for 1,000 yd (910 m)
• Propulsion : Compressed air

Mark VII and VII*

Introduced on the 1908 members of the 1905 Tribal class destroyers. Used by torpedo boats built before the First World War and destroyers. Used by RAF flying boats in the 1920s.

• Year : 1907^[1]
• Warhead : 320 lb (150 kg) TNT
• Performance :
  • Mark VII :
    30 knots (56 km/h; 35 mph) for 6,000–7,000 yd (5,500–6,400 m)
    41 knots (76 km/h; 47 mph) for 3,000 yd (2,700 m)
  • Mark VII* :
    29 knots (54 km/h; 33 mph) for 7,000 yd (6,400 m)
    35 knots (65 km/h; 40 mph) for 5,000 yd (4,600 m)
• Propulsion : Wet-heater

Mark VIII

• Year: 1913
• Role: Submarines and aircraft (Note: During World War II and after the Mk.VIII was a 21-inch torpedo)
• Warhead: 320 lb (150 kg) TNT
• Propulsion: Wet heater
• Performance: 35 knots (65 km/h; 40 mph) for 2,500 yd (2,300 m)

Mark XI

• Year: 1934
• Aircraft carried
• Dimensions: 17.72 in (450 mm) dia.
• Warhead: 465 lb (211 kg) TNT
• Propulsion: Burner cycle
• Performance: 35 knots (65 km/h; 40 mph) for 2,500 yd (2,300 m)

Mark XII

18-inch Mark XII
A Mark XII torpedo fitted to a Bristol Beaufighter
Type	Torpedo
Place of origin	United Kingdom
Service history
In service	1937-1945
Production history
Designed	1935
Specifications
Mass	1,548 lb (702 kg)
Length	16 ft 3 in (4.95 m)
Diameter	17.72 in (450 mm)
Warhead	TNT
Warhead weight	388 lb (176 kg)
Engine	Burner cycle 140 hp (at 40 knots)
Maximum speed	40 knots (74 km/h; 46 mph) for 1,500 yd (1,400 m) or 37 knots (69 km/h; 43 mph) for 3,500 yd (3,200 m)

Aircraft launched, used by Fleet Air Arm and RAF Coastal Command.

Mark XIV

The Mark XIV was an aircraft-launched torpedo. Stocks were lost with the fall of Singapore.

Mark XV

• Year: 1942
• Operators: Fleet Air Arm, RAF Coastal Command, Royal Navy
• Role: Aircraft and Motor Torpedo Boats, 1943 onwards
• Dimensions: 17.69 in (449 mm) × 17 ft 2.7 in (5.250 m)
• Warhead: 545 lb (247 kg) Torpex
• Propulsion: Burner cycle
• Performance: 40 knots (74 km/h; 46 mph) for 2,500 yd (2,300 m) or 33 knots (61 km/h; 38 mph) for 3,200 yd (2,900 m)^[2]

Mark XVI

Electric torpedo project not completed.

Mark XVII

• United Kingdom, 1944
• Aircraft launched
• Operated by FAA, RAF Coastal Command, Blackburn Firebrand
• Dimensions: 17.72 in (450 mm) × 17 ft 3 in (5.26 m)
• Warhead: 600 lb (270 kg) Torpex
• Propulsion: Burner cycle
• Performance: 40 knots (74 km/h; 46 mph) for 2,500 yd (2,300 m)

Mark 30

An air-dropped passive acoustic homing torpedo known as "Dealer" and "Dealer B".^[3]

• Length: 8 ft (2.4 m)^[4]
• Weight: 646 lb (293 kg)
• Performance: 12.5 knots (23.2 km/h; 14.4 mph) for 3,000 yd (2,700 m) or 19 knots (35 km/h; 22 mph) for 830 yd (760 m)